Various power devices, such as field effect transistors (FETs) and insulated gate bipolar transistors (IGBTs), require relatively efficient heat sinking to eliminate performance problems attributable to thermal limitations. Historically, these devices have been packaged in, for example, TO-220, DPAK or D2PAK type semiconductor packages, with the devices being mounted to a copper slug and then overmolded. Such semiconductor packages have then been mounted to a highly thermally conductive substrate or heat rail, which provides for one-sided heat sinking.
An alternative method has mounted a flip chip die directly to a substrate with a back of the die being coated with a thermal interface material to engage a fixed pedestal heat sink that is fabricated as part of a case. Dual-sided heat sinking in this manner generally reduced thermal resistance by an order of magnitude. In such a configuration, it is highly desirable to minimize the gap between the pedestal and the die as the thermal interface material positioned in the gap between the pedestal and the die has generally had significantly higher thermal resistance than the other components.
In one such electronic module, a relatively thin (0.031 inch) flexible laminate substrate was used to improve alignment of a die with a fixed pedestal heat sink. Due to the fact that the substrate had limited compliance, the force applied to solder joints of the die was frequently asymmetrical. This asymmetrical force has tended to increase the variability of solder joint reliability. Further, the variation in the gap between the die and the fixed pedestal heat sink has resulted in a variable thermal performance between similar assemblies. Additionally, utilizing a fixed pedestal heat sink limits the use of a rigid substrate and, thus, makes solder joints that attach a die to a substrate more vulnerable to cracking prior to underfill.
FIG. 1 depicts a partial cross-section of a typical prior art electronic module 10 that includes a case 12 having an upper half and a lower half. The upper half of the case 12 includes an integrated fixed pedestal heat sink 14, which is in thermal contact with a flip chip 16. As is shown, a thermal interface material 18 is located at the interface between the heat sink 14 and the flip chip 16. As shown, the flip chip 16 includes a number of contacts 20 that are soldered to traces 22 formed on a substrate 24. The substrate 24 is mounted to the upper portion of the case 12 via a standoff 27 and an elastomer 26 supports the substrate 24 and keeps the material 18 in compression to ensure thermal contact is maintained between the flip chip 16 and the heat sink 14. The module 10 requires relatively tight tolerances for flip chip 16 tilt and substrate 24 warpage and requires that the substrate 24 be relatively flexible to allow the flip chip 16 to align to the fixed pedestal heat sink 14, which places solder joints at risk to crack prior to stabilization. An underfill 28 stabilizes the flip chip 16. As previously discussed, due to uneven application of force to the flip chip 16, the reliability of the flip chip 16 is somewhat variable as asymmetrical loads on the solder joints of the flip chip 16 can result in failure of the solder joints.
What is needed is an improved technique for conducting heat away from an integrated circuit (IC) package.